Peter Frach, Daniel Gloess, Thomas Goschurny, Andy Drescher, Ullrich Hartung, Hagen Bartzsch, Andreas Heisig, Harald Grune, Lothar Leischnig, Steffen Leischnig, Carsten Bundesmann
Pulse magnetron sputtering is very well suited for the deposition of optical coatings. Due to energetic activation during film growth, sputtered films are dense, smooth and show an excellent environmental stability. Films of materials like SiO2, Al2O3, Nb2O5 or Ta2O5 can be produced with very little absorption and scattering losses and are well suited for precision optics. FEP's coating plant PreSensLine, a deposition machine dedicated for the development and deposition of precision optical layer systems will be presented. The coating machine (VON ARDENNE) is equipped with dual magnetron systems (type RM by FEP). Concepts regarding machine design, process technology and process control as well as in situ monitoring are presented to realize the high demands on uniformity, accuracy and reproducibility. Results of gradient and multilayer type precision optical coatings are presented. Application examples are edge filters and special antireflective coatings for the backlight of 3D displays with substrate size up to 300 x 400mm. The machine allows deposition of rugate type gradient layers by rotating a rotary table with substrates between two sources of the dual magnetron system. By combination of the precision drive (by LSA) for the substrate movement and a special pulse parameter variation during the deposition process (available with the pulse unit UBS-C2 of FEP), it is possible to adjust the deposition rate as a function of the substrate position exactly. The aim of a current development is a technology for the uniform coating of 3D-substrates and freeform components as well as laterally graded layers.
Aluminum nitride (AlN) is a piezoelectric material often used as thin film in SAW/BAW devices. Furthermore, there is an increasing interest in its use for energy harvesting applications. Despite it has a relatively low piezoelectric coefficient, it is a suitable choice for energy harvesting applications and due to its low dielectric constant and good mechanical properties. In addition, it is a lead-free material. The films were deposited by reactive pulsed magnetron sputtering using the Double Ring Magnetron DRM 400. This sputter source together with suitable powering and process control allows depositing piezoelectric AlN very homogeneously on 8” substrates with deposition rates of up to 200 nm/min. With the developed technology, film thicknesses of several ten microns are technically and economically feasible. Moreover, by adjusting process parameters accordingly, it is possible to tune properties, like film stress, to application specific requirements. Additionally, it is known that the doping of AlN with Scandium results in a significantly increased piezoelectric coefficient. The influence of process parameters and Sc concentration on film properties were determined by piezometer, pulse echo, SEM, XRD, EDS and nanoindentation measurements. Energy harvesting measurements were done using an electromechanical shaker system for the excitation of defined vibrations and a laservibrometer for determination of the displacement of the samples. The generated power was measured as function of electric load at resonance. An rms power of up to 140μW using AlN films and of 350μW using AlScN films was generated on Si test pieces of 8x80mm2. Furthermore, energy harvesting measurements using manually bended steel strips of 75x25mm2 coated with AlScN were carried out as well. When using only a single actuation, energy of up to 8μJ could be measured. By letting the system vibrate freely, the damped vibration at resonance 50Hz resulted in a measured energy of 420μJ.
This paper reports on the deposition of AlN and AlXSc1-XN films by pulse magnetron sputtering. The influence of process parameters on the film properties and the evaluation of the films for micro energy harvesting are presented. For AlN it is shown, that film stress can be varied in a considerable range between compressive and tensile stress while maintaining good piezoelectric properties. Additionally, the effect of doping AlN with Sc regarding piezoelectric and mechanical properties is presented. The films show the expected increase of piezoelectric properties as well as the softening of the material with higher Sc concentrations. Above a threshold concentration of around 40% Sc in the AlXSc1-XN films, there exists a separation into two phases, an Al-rich and a Sc-rich wurtzite phase, which is shown by XRD. At Sc concentrations higher than 50%, the films are not piezoelectric, as the films are composed primarily of the cubic ScN phase. Sc doping allows to significantly increase the energy generated in test setup. Up to 350 μW power have been generated under optimum conditions.
Aluminum nitride is a promising material for the use as a piezoelectric sensor material for resonance frequencies higher
than 50 MHz and contains the potential for high frequency phased array application in the future. This work represents
the fundamental research on piezoelectric aluminum nitride films with a thickness of up to 10 μm based on a double ring
magnetron sputtering process.
The deposition process of the aluminum nitride thin film layers on silicon substrates was investigated and optimized
regarding their piezoelectric behavior. Therefore a specific test setup and a measuring station were created to
characterize the sensors. Large single element transducers were deposited on silicon substrates with aluminum
electrodes, using different parameters for the magnetron sputter process, like pressure and bias voltage. Afterwards
acoustical measurements were carried out in pulse echo mode up to 500 MHz and the piezoelectric charge constants (d33)
were determined. As a result, two parameter sets were found for the sputtering process to obtain an excellent
piezoelectric charge constant of about 7.2 pC/N maximum.
Magnetron sputtering of a silicon target in a time-variant mixture of the reactive gases oxygen and nitrogen allows the
deposition of optical multilayer and gradient layer systems of silicon oxinitride at one stationary sputtering station. In
this paper the processes within the sputter discharge and the properties of the growing film during the change of the
reactive gas composition are investigated using optical in-situ monitoring, optical plasma emission spectroscopy and
plasma impedance monitoring. A time delay between the change to the reactive gas composition and the resulting change
to the film composition was observed. The time delay is longer for the transition from oxide to nitride deposition then
vice versa. This asymmetry is attributed to the different affinity of nitrogen and oxygen to the silicon target. Examples of
deposited antireflective coatings as well as rugate filters based on silicon oxinitride multilayer and gradient layer designs are given.
Evaporation and reactive pulse magnetron sputtering are two methods to deposit broadband antireflection coatings at low temperatures. The performance of broadband antireflective coatings on polycarbonate (PC) deposited by these methods is shown. Plasma-treatment of the plastic substrates has an important effect on the adhesion of the coating system on the substrate. For sputtering different parameter sets for the pre-treatment as well as the deposition pressure and their effect on adhesion were examined for polycarbonate substrates. Furthermore evaporation and reactive pulse magnetron sputtering were compared with regard to the adhesion of broadband antireflective coatings on PC.
Optical coatings have been deposited by pulse magnetron sputtering of the target materials silicon and aluminium using the reactive gases oxygen, nitrogen and fluorine. The measured refractive index and roughness of the SiO2, Si3N4, Ta2O5 and Al2O3 films indicate the deposition of very dense and smooth films. Absorption of SiO2 and Al2O3 films is low even at a wavelength of 193nm. The sputter deposited AlF3 layers have low absorption until 150 nm but then show an absorption edge. Examples of multilayer include antireflective coatings for ophthalmic lenses and cut filters consisting of up to 150 λ/4 layers. These films were produced at a single stationary coating station without interruption of film deposition by changing the reactive gas during the plasma process. This process was also used for the deposition of rugate filters with apodisation function by continuous variation of the reactive gas composition during the deposition.
Optical inhomogeneous gradient films have been produced by reactive pulse magnetron sputtering of a silicon target in a variable mixture of the reactive gases oxygen and nitrogen. The presented coating system allows the automatic deposition of the gradient films with predefined, freely varying refractive index at a deposition rate of 1 nm/sec uniformly across 8” diameter.
This system has been used for deposition of antireflective coatings, rugate filters, edge filters and dichroic filters composed of SixOyNz films with both gradient and sharp transitions between oxide and nitride. The deposited antireflective coatings consist of one gradient and one homogeneous layer and have a reflectivity of less than 0.5% in the wavelength range between 440 and 620 nm. Thermal substrate load during deposition of these coating is considerably lower than for deposition of conventional multilayer of silicon and titanium oxide. An example of a deposited simple narrow band rugate filter has 45 cycles of sinusoidal refractive index modulations between 1.67 and 1.95, resulting in a reflectivity of > 99.9% at 550 nm with a half-power bandwidth of 42.5nm and an average transmission of 95% in the remaining visual spectral range. The produced edge and dichroic filters consist of quarter-wave oxide and nitride layers. All films have been deposited at one sputtering station without interruption of the plasma thus guaranteeing a very stable, reproducible and highly efficient process.
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